1. Field of the Invention
The present invention relates to an image reading apparatus such as a scanner or a facsimile apparatus.
2. Related Background Art
As shown in the sectional view of FIG. 1 and the plan view of FIG. 2, an image sensor used in an original reading apparatus comprises a sensor IC 1, a sensor board 2 on which the sensor IC 1 is mounted, a lens array 3, an illumination unit 4, a cover glass 5, and a frame 6 for positioning/holding these members. The illumination unit 4 comprises lead frame type LEDs 7 serving as light sources, a light guide 8, and a housing 9 for positioning/holding the LEDs 7 and the light guide 8.
FIGS. 3A and 3B are sectional and side views, respectively, showing the detailed structure of the illumination unit 4. Referring to FIGS. 3A and 3B, the illumination unit 4 is constituted by the lead frame type LEDs 7 serving as light sources, the columnar light guide 8 consisting of a transparent member of, e.g., an acrylic resin, and the housing 9 which has a hexagonal section to position/hold the LEDs 7 and the light guide 8.
The LEDs 7 are arranged at the two ends of the light guide 8 such that light beams are incident into the light guide 8. The light guide 8 has a diffusion surface 10. Fine tapered portions are continuously formed on the diffusion surface 10 along the longitudinal direction of the light guide 8, as shown in FIG. 4. A light beam incident on this tapered portion at an incident angle .theta. of 41.degree. or more (when the light guide 8 consists of an acrylic resin and has a refractive index n of 1.5) satisfies the total reflection angle condition so that the light beam emerges in a desired direction.
The housing 9 for positioning/holding the LEDs 7 and the light guide 8 has, on a surface corresponding to one of the six sides of its hexagonal section, a slit 12 for passing light in the exit direction. To ensure a light amount, the light guide 8 preferably has a white-based color with a high light reflection efficiency. The light guide 8 is formed of, e.g., an ABS resin.
The LEDs 7 are electrically connected to an external system through connector cables 16. Since two LEDs 7 sandwich the light guide 8, two connector cables 16 are necessary. Cables 16 are used to connect leads 11 of the LEDs 7 to driving unit 18, which is used to turn the light source on.
In the above prior art, the LEDs 7 are electrically connected to the external system through the connector cables 16. For this reason, the following problems are posed.
(1) The number of connector cables 16 equal the number of LEDs 7. The cost of the connector cables 16 and an increase in labor for attaching the connector cables 16 to leads 11 of the LEDs 7 result in an increase in cost.
(2) Electrical connection between the image sensor and the external system is complex and therefore easily affected by noise.
(3) A predetermined illuminance on the original read line can hardly be obtained because no luminance adjusting circuit for the LEDs 7 can be inserted between the LEDs 7 and the external system. For this reason, a sensor output ("bright" sensor output) in reading, e.g., a white original largely varies between image sensors as end products.